The Broward Sheriff Office confirmed at 4:11 p.m. ET that the shooter had been apprehended and taken into custody.
"I don't know the number that's been transported at this point, but there are numerous fatalities," Runcie told reporters. "It's a horrific situation."
Police have not released the identity of the shooter, though Runcie said it is "potentially" a former student of the high school.
"Yes, it is potentially some former student," Runcie said. "That individual, we believe, is in custody at the moment. At this point, we have no evidence that there's been more than one shooter."
At least 14 victims have been confirmed by the Sheriff's Office as of 3:22 p.m. ET.
"So far we have at least 14 victims," the office tweeted. "Victims have been and continue to be transported to Broward Health Medical Center and Broward Health North hospital."
So far we have at least 14 victims. Victims have been and continue to be transported to Broward Health Medical Center and Broward Health North hospital. #StonemanShooting— Broward Sheriff (@browardsheriff) February 14, 2018
A student told CNN affiliate WSVN that the fire alarm at the high school was pulled just before dismissal, and the shooting began shortly after.
"We all thought it was a fire drill because we had one previously today," the source said. "And we thought it was, so no one was that nervous, but then word started going around that it was shots and not just, like, something else, everyone just started running towards the canal."
President Donald Trump commented on the shooting minutes after the initial report, offering his condolences.
"My prayers and condolences to the families of the victims of the terrible Florida shooting," Trump tweeted. "No child, teacher or anyone else should ever feel unsafe in an American school."